A PWM control signal is a discrete signal exhibiting edges upon each change of state. A control circuit switches upon each edge of the control signal. Each switching operation causes a sudden current variation or edge and a sudden voltage variation or edge. A sudden current variation or edge causes interference in the form of a conducted emission. A sudden voltage variation or edge causes interference in the form of a radiated emission. The higher the slew rate, or slope, of the current or voltage, the more substantial, and hence harmful, the interference.
It is known practice, in order to decrease conducted interference, to decrease the slew rate of the current and, in order to decrease radiated interference, to decrease the slew rate of the voltage.
The slew rate of the current may be decreased by increasing the capacitance at the input of the voltage source, typically by replacing the input capacitor. An increase in this capacitance leads to a detrimental increase in cost.
Radiated interference following variations in voltage may be decreased by acting upon the cable bundle: by twisting the wires of the cable bundle or by implementing shielding. Carrying out such twisting leads to a detrimental increase in cost. Moreover, such an operation is rarely the preserve of the control circuit manufacturer, which does not necessarily specialize in cable bundles.
The lower the slew rate, the lesser the interference. However, the lower the slew rate, the higher the dissipated power. An increase in dissipated power leads to an increase in the size of components and thus to a detrimental increase in cost.
In addition, when multiple control circuits operate together, at least two such control circuits may advantageously be synchronized, so that at least some current or voltage edges or sudden variations therein are made to coincide in order to compensate for, or even cancel out, the interference resulting from these edges. Such synchronization is particularly advantageous when the respective control signals of the control circuits have one and the same period.
Trivially, it appears to be possible to synchronize at least two control circuits, by directly synchronizing the edges of the control signals. However, such an approach does not meet its objective in that, due to variations in the electrical characteristics of the components, thermal drift, variations in the applied voltage, or else variations in the current level, the interval between one edge of the control signal and one current or voltage edge exhibits dispersion that is sufficiently substantial, from one control circuit to another, for the interference not to be synchronized enough to allow compensation.